1. Field of the Invention
The present invention relates to a circuit arrangement having at least one switching device and a free-wheel diode connected in parallel with the switching device. The present invention is particularly useful when it is applied to a power semiconductor module having a rectifying device.
2. Description of the Related Art
Semiconductor power modules are used in various fields as a device that composes an inverter. Particularly, power modules that use a Si-IGBT (Insulated Gate Bipolar Transistor) as a switching device and a Si-PiN diode (hereinafter referred to as a Si-PND) as a free-wheel diode exhibit low loss and high blocking voltage and are used in a wide variety of fields such as railroads and consumer electronics. In recent years, energy savings have become increasingly important. Therefore, it is demanded that the power modules exhibit lower loss. The loss of a power module is determined by the performance of an employed power device. The Si-IGBT has improved its performance year after year, whereas the Si-PND has not made a major breakthrough. Current diodes suffer a recovery current problem in which carriers stored in the diodes are discharged upon IGBT turn-on. This problem not only brings about an increased switching loss but also causes noise generation. Therefore, diodes with a minimum of recovery current are highly demanded. However, a region where the characteristics of the Si-PND are substantially determined by the material properties of Si is already reached. It is therefore difficult to greatly reduce the recovery current. One of some previously developed technologies for recovery current suppression provides the anode surface of a PiN diode (PND) with a region having a Schottky interface to restrict minority-carrier injection. An example of a PND having a Schottky region is disclosed in Japanese Patent No. 2590284.
On the other hand, power devices based on silicon carbide (SiC) are expected to exhibit higher performance than Si-based power devices due to excellent physical properties of SiC. Since SiC has high breakdown field strength, the thickness of a SiC-based device can be considerably smaller than that of a Si-based device. Therefore, even a unipolar SiC device can simultaneously exhibit high blocking voltage and low resistance upon power-on. Further, even if a bipolar SiC device is used, the thickness of the device can be small so that switching characteristics improve due to a decrease in the number of carriers stored in the device. Among SiC devices, diodes are more advanced in terms of low specific on-resistance and large capacity than switching devices. Therefore, an attempt is being made to achieve low loss by combining a Si-IGBT with a SiC diode. A combination of a Si-IGBT and SiC diode is described in JP-A-2006-149195.
A SiC diode differs from a Si diode in that the former permits a Schottky barrier diode (hereinafter referred to as the SBD) to exhibit a blocking voltage higher than 3 kV. Therefore, the SBD and PND can be selectively used depending on the blocking voltage class. The SBD is used in a low blocking voltage region because it has a lower built-in potential than the PND and reduces the forward voltage upon rated current application. Further, since it is a unipolar device, it can remarkably reduce the recovery current prevailing upon IGBT turn-on. However, as the recovery current is reduced to substantially zero, the electrical current sharply changes. This causes the capacitance and inductance components in a circuit to resonate, thereby generating switching noise. The noise may not only damage the device but also make the entire system faulty. Furthermore, the SBD cannot permit a large current to flow in marked contrast to the PND. Therefore, the SBD might be damaged by a momentary large current called a surge. On the other hand, the PND has a high built-in potential so that the forward voltage prevailing upon rated current application is high in a low blocking voltage region. However, since the PND is a bipolar device, the voltage increase due to the thickness of a drift layer is limited. In a high blocking voltage region, therefore, the PND is lower than the SBD in the forward voltage prevailing upon rated current application. In addition, the PND has high resistance to a surge because it permits a large current to flow. As the SBD and PND have their own advantages and disadvantages as described above, they should be selectively used to achieve the intended purpose.
Meanwhile, a structure called an MPS (Merged PiN Schottky) was recently proposed as a device that is obtained by combining the above two diodes. This structure has both a PN junction region and a Schottky junction region on the anode side. In a normal operating region, the Schottky junction region mainly works. When a surge current flows, the PN junction region operates for device protection. Further, this structure can suppress a leak current from a Schottky junction because, when a reverse bias is applied, a depletion layer extends from the PN junction region to prevent the Schottky junction region from being exposed to a high electric field. An example of the MPS is disclosed in “2nd Generation SiC Schottky Diode: A New Benchmark in SiC Device Ruggedness” (Proceedings of ISPSD 2006, 305).